In the related art, there is a method of deriving an optical parameter of an anisotropic material. For example, Y. Kim et al., “Investigation of THz birefringence measurement and calculation in Al2O3 and LiNbO3”, Appl. Opt. 50, 2906, (2011) discloses a method of acquiring a temporal waveform of a terahertz pulse for each rotation angle of an anisotropic material and deriving a complex refractive index of each axis after an orientation of the axis is determined. S. Katletz et al., “Polarization sensitive terahertz imaging: detection of birefringence and optical axis”, Opt. Express 20, 23025, (2012) discloses a method of probing an anisotropic material using a terahertz pulse with circular polarization and deriving a birefringence parameter by analyzing a result of probing using a predetermined calculation equation. Japanese Unexamined Patent Publication No. 2016-99309 discloses a method of directly detecting an electric field vector of a terahertz wave by using an electro-optical crystal obtained by cutting out a (111) plane of an optically isotropic medium as a terahertz wave detection element and performing probing with a terahertz wave by setting probe light that is ultrashort pulse light to circular polarization.